Cellular Reproduction, Part 1: Mitosis Lecture 10 Fall 2008

1. Cellular Reproduction, Part 1: Mitosis Lecture 10Fall 2008

2. 1 Cell Theory Cell theory: • All organisms are made of cells • All cells arise from preexisting cells How do new cells arise? • Cell division • the reproduction of cells

3. 2 Cell Division What is cell division used for? • Growth • Wound repair • Reproduction Both sexual and asexual

4. 3 Cell Division • Multicellular organisms start out life as a single cell • A series of cell divisions occur that give rise to all the cells in an organisms body Fig. 8.1

5. 4 Cell Division Two components • Cytokinesis (“cell movement”) • Division of the cytoplasm into two daughter cells • Division of the nucleus (and of the DNA)

6. 5 Cell Division Two different types of nuclear division possible Mitosis • Form of cell division that leads to identical daughter cells with the full complement of DNA • Occurs in somatic cells • Cells of body that are not reproductive cells Meiosis • Form of cell division that leads to non-identical daughter cells with one-half the complement of DNA • Forms gametes • Reproductive cells (sperm cells & egg cells)

7. 6 Chromosomes • Genome • the complete complement of an organisms genetic material, DNA • Gene • a discrete unit of hereditary information consisting of a specific nucleotide sequence in DNA • Prokaryotes have one long DNA molecule • Eukaryotes have many DNA molecules = chromosomes • Chromosome: a thread-like, gene-carrying molecule located in the nucleus

8. 7 Chromosomes • Chromosomes in the nucleus contain most of an organisms genes • Some DNA in mitochondria and chloroplasts • Number of chromosomes vary depending on species • Somatic cells of humans = 46 chromosomes • Two sets of 23 chromosomes • 23 from egg and 23 from sperm • Gametes of humans = 23 chromosomes

9. 8 Chromosomes • Chromosomes made of chromatin • Complex of DNA and protein molecules • Role of protein molecules • Organize chromatin • Help control activity of gene • Mass of long fibers spread out in the nucleus • Chromatin fibers form compact chromosomes only during cell division • Each chromosome containsone very long, linear DNA moleculethat carries several hundred to a few thousand genes Fig. 12.3

10. 9 DNA Packing Chromosomes condense to structures visible with a light microscope • Histones attach to DNA • Histones – small proteins that assist with DNA packing • Creates an area called a nucleosome • DNA wrapped around histones • “beads on a string” • Tight helical fiber • Supercoil • More looping & folding Fig. 16.21

11. 10 Replication of Chromosomes DNA needs to be duplicated prior to cell division • Duplicated chromosome = sister chromatids • Held together by adhesive proteins (cohesins) • Centromere • Specialized region where sister chromatids most closely attached • During mitosis, sister chromatids separate, one each to the two daughter cells • Once separated, they are now individual chromosomes Fig. 12.4

12. 11 The Cell Cycle Cell cycle • Life cycle of the cell • Starts with origination of a cell and ends with cell division • Two main phases • Interphase • ~90% of time • Mitotic phase • ~10% of time Fig. 12.5

13. 12 The Cell Cycle Interphase • Normally activity of cell • Cell grows as it produces more contents of cell cytoplasm • Chromosome duplication occurs Fig. 12.5

14. 13 The Cell Cycle Interphase • Cell growth occurs in all three sub-phases • G1 (first gap) • S phase (synthesis) • Chromosome replication • Sister chromatids • G2 (second gap) • Cell preparing to divide Fig. 12.5

15. 14 Mitosis Mitotic phases • Prophase • Prometaphase • Metaphase • Anaphase • Telophase • Cytokinesis occurs during telophase

16. Activity

17. 15 G2 of Interphase • Chromosomes (chromatin) duplicated • Chromosomes still uncondensed • Not clearly distinguishable as separate chromosomes • Centrosome • Non-membrane bound organelle • Organizes the cell’s microtubules • Centrosome replicates into 2 • Contains centrioles • Structures made of microtubules • Found only in animals • Microtubules will form even if centrioles destroyed Fig. 12.6

18. 16 Mitosis: Prophase • Chromosomes condense • Each chromosome appears as two identical sister chromatids joined together • Formation of mitotic spindle • Centrosomes move towards poles of cell • Microtubules extend from centrosomes • Asters – short microtubules extending from centrosome • Will guide the separation of the daughter chromosomes Fig. 12.6

19. 17 Mitosis: Prometaphase • Breakdown of nuclear envelope • Microtubules of mitotic spindle start to attach to chromosomes • Centromere contains special attachment proteins • kinetochore • Movement of chromosomes towards center of cell • Non-kinetochore microtubules interact with others from opposite pole Fig. 12.6

20. 18 Mitosis: Metaphase • Centrosomes at opposite poles • Mitotic spindle fully formed • Chromosomes lined up at equator of cell (metaphase plate) • Each sister chromatid is attached to a kinetochore microtubule • Other non-kinetochore microtubules meet with each other from opposite ends of the cell Fig. 12.6

21. 19 Mitosis: Anaphase • Sister chromatids of each chromosome separate • Cohesion proteins cleaved • Now a “daughter” chromosome • Chromosomes move away from center, towards poles • Kinetochore microtubules shorten & bring chromosomes with them towards each pole • Read Fig. 12.8 Inquiry • Cell elongates • Non-kinetochore microtubules lengthen • Pushes poles of cell further apart • End of Anaphase • Both poles have equivalent and complete set of chromosomes Fig. 12.6

22. 20 Mitosis: Telophase • Chromosomes at poles of cell • Two daughter nuclei begin to form in the cell • Nuclear envelope forms around each one • Chromosomes become less condensed • Mitotic spindle deconstructed Fig. 12.6

23. 21 Cytokinesis Cytokinesis in animals • Cleavage • Cleavage furrow • Shallow groove at equator of cell • Parent cell “pinched” in two by contraction of ring of microfilaments Fig. 12.9

24. 22 Cytokinesis Cytokinesis in plants • Cell wall material bound in vesicles • Vesicles line up, fuse to form cell plate • Grows until it fuses with plasma membrane • Cell wall contents bind with cell wall Fig. 12.9

25. 23 Asexual Reproduction • Mitosis also used to reproduce entire organisms • Asexual reproduction • Reproduction involving only one parent that produces genetically identical (clone) offspring • Does not involve sperm and egg (no meiosis) Fig. 8.2

26. 24 Asexual Reproduction Japanese knotweed broken stem sprouts shoots and roots Aspen root “suckers” – produce new shoots and stems

27. 25 Asexual Reproduction Binary fission • Prokaryotes • Not mitosis • Single large circular chromosome • Chromosome duplicated • Starts at origin of replication • Copy of origin site moves to other pole of cell • Cell elongates • Plasma membrane grows inward to form two daughter cells Fig. 12.11

28. 26 Evolution of Mitosis • Hypothesis: mitosis evolved from binary fission in prokaryotes • Some similar proteins • Possible intermediate stages still seen other organisms Fig. 12.12